IP Precedence and DSCP Values

This topic is to discuss the following lesson:

Great article and it makes a lot of sense about DSCP and IP Prec values.

Hi Hamood,

Glad to hear it was useful!


Very Nice article and very well framed.
Kudos to Rene.


Can you explain me about Quality of Service on trunk links? It’s important.


Hi Alberto,

I can, anything in particular you want to know?


Hi Rene

Abut 802.1P and how configure It.

Hi Alberto,

You can take a look here:


Only thing I should explain in another post are the “mls qos maps”. The switch will overwrite the DSCP value in the IP packet based on the COS value.

As always. Great article and lots way more easier to understand that vendor’s documentation.

Very good Explanation :slight_smile:

Hi Rene, that is great stuff. I never really understood QoS but this article helped me to get the basics. Thank you!

Hi Rene, Another great explanation, kudos ! i have 2 doubts though :

First one may seem stupid but i will ask:-)

  1. When we say , audio traffic / video traffic and talk about assigning them separate DSCP value based on our priority - how do we achieve that, because ultimately both type of traffic is coming from same host / IP Address and if I am not wrong , underlying we use IP address to create class-map/policy-map etc from config pint of view. so if IP is same from where audio/video traffic being generated, how do we separate them at switch for assigning different dscp values ?

  2. Now in real world as a network engineer, if we are asked to design QoS to make sure our voice/video application (Skype for business in my case) is working perfectly fine in the network consist of Cisco switch / router
    what parameters / configs/ concept i need to worry ? a holistic answer of it will really help.


I would recommend you check out Rene’s lesson on QoS Classification. In this lesson, you will discover that there are all kinds of tools at your disposal to be able to identify, classify, and mark traffic for QoS treatment throughout your network. It is rarely the case, for the reasons you brought up, that a simplistic approach of using just IP addresses is used.

As for your second question, you are essentially asking about what QoS is meant to do as a whole (so there isn’t a quick, easy answer). The key to getting QoS working properly relies on the network engineer doing a few critical things before actually configuring anything:

  1. Work with your business units to learn network expectations, and what applications are important to them
  2. Determine a kind of SLA from the business units to gauge how they want their applications to perform under heavy load
  3. Perform an “inventory” of what kind of traffic is present on your network, as well as the patterns of the traffic. For example, does it spike at certain times of the day? How do these spike times line up with how the rest of the business is using the network?

After you have answered questions like these, then the actual QoS work begins. In a nutshell, you need to identify and mark important kinds of traffic as close to the source as possible. Once the traffic is marked, you can configure the devices on your network to treat this traffic in a way that is consistent with the expectations you learned from your meetings with the business units.

As in example in your case, you could use NBAR or a combination of IPs and ports to classify the Skype traffic. You would have this traffic assigned a high priority marking (like DSCP EF, for example for voice data), then configure queuing (perhaps LLQ) to ensure this traffic is processed quickly under load.

As you can see, QoS is a complicated topic–it will take a lot of reading and study on your part to master it!

Thanks a lot Andrew, your answer gives a good direction to proceed. thx again !


Could i get clarification on few statements :

A. “There are 4 different classes and each class will be placed in a different queue” AND “Class 4 has the highest priority”. please help in understand this statement as why class 4 has highest priority - i could’nt see any such mapping in table given.

B. why under class-1 we have 001010 and how it is coming as AF 11 and then same way under class 2 we have 010010 and it is AF 21…so on. how is this mapping.


Hi Abhishek,

There are four classes: 1, 2, 3 and 4. The higher the class number, the higher the priority. Class 4 has a higher priority (value) than any of the other classes.

Within each class, there is a low, medium and high drop probability.

These values are described in the RFC. In reality, they are just markings and it’s up to you to configure your routers/switches to act upon these different markings.


Thanks Rene !

awesome, it clears now. Thanks for your crystal clear approach of explanation.

I am eagerly waiting for L2VPN related topics (vpls etc…) :slight_smile:

shouldn’t the last class-selector name be cs7 instead of cs6?

Hi James,

For sure! I see I had CS6 twice, just fixed it. Thanks for letting me know.